When you create a SharePoint calendar, by default, you can’t add attendees to events:
To add the attendees part in this windows, follows these steps:
- Go to List Settings in the Calendar tab.
- In the Content Types part, click on Event.
- In the Columns part, click on Add from existing site or list columns.
- Select Attendees and click on Add >.
Now, if you go back to your calendar, and if you create/edit an event, you will be able to add attendees to this item: